This invention relates to an apparatus for filtration of molten metals, and in particular, to such an apparatus adapted for filtration of molten metals as they are poured into a mold. More specifically, this invention relates to a hollow frusto-conical strainer having a plurality of slot apertures of uniform width for separating particulant impurities from the molten metal. The strainer may rest within a pour cup having an outlet port which mates with the inlet port of a mold. The slot apertures are disposed longitudinally about the conical surface of the strainer and are of staggered lengths so that there are relatively fewer slots extending to the smaller diameter portion of the strainer than extending to the larger diameter portion of the strainer. The bottom of the strainer has a plurality of apertures of a size substantially equal to the width of the slot apertures. The top of the strainer is open and has a flange disposed about its perimeter which abuts the lip of the pour cup. The strainer and the pour cup are made of refractory or ceramic material of sufficient heat resistance and strength to withstand the heat and thermal shock of molten metal.
Filters for molten metal have long been known in the art, but their usefulness has been limited by their substantial restriction of metal flow rate, the difficulties encountered in their use and/or their tendency to introduce unwanted foreign material into the mold. Presently used filters generally consist of disk-like members which are cemented or grouted into the inlet port of the mold. Unless the cementing or grouting is done with extreme care, it is possible for the molten metal to bypass and flow around the filter element. Further, during insertion, the grout or cement material may enter the mold as fine particles which cannot be removed once the filter is in place. Accordingly, nonmetallic inclusions can be trapped inside the mold and contaminate the casting. Three types of disk-like filters are commonly used. The first, known as a strainer core, is made by pressing a plate of ceramics with a series of holes. Because of the limitation of the pressing process, it is usually not possible to obtain greater than a 50 percent open area of holes. This results in a large restriction of metal flow making a strainer core unsuitable for investment castings. The second type of filter is a ceramic foam having an open cell structure. As in the case of a strainer core, this type of filter cannot be used for investment casting, since the flow restriction is too great. The third type of filter has a screen-like appearance and is made by extruding ceramic. The open area can be as high as 70 percent, and accordingly, this type of filter has become popular in investment casting. However, because this product is flat and light, the limitations inherent in cementing or grouting the filter into the mold inlet port as described above preclude its full acceptance.